Battery disconnect

Today I disconnected the negative battery terminal to wire up a new reaburies mirror. Upon reconnecting it I noticed the stations were still there but the clock needs to be reset. I ordered a new steering wheel with radio controls and everything I read and watched said to disconnect the neative terminal. I afraid to do this mod now in fear that there still maybe becharge and the air bag going off on me. Can any one shed some light on to why the radio stations don't need reset but the clock does. And am I corrct that just disconnecting the negative terminal will do.
Thanks
Steve